The invention relates to a method and apparatus for improving the sharpness of television pictures and in particular to use of a modulated sharpness signal which is added to the television signal in order shorten transition times in the television signal while minimizing background noise.
It is known in the art that the transitions occurring in a video signal can be shortened in order to compensate for frequency response degradation during processing, transmission or recording of the signal. In prior art sharpening circuits, the amplitude of the video signal is boosted at mid to high frequencies, or an auxiliary signal derived from the video signal is added to the video signal after correction for phase and delay. Such known circuits are exemplified by the teachings of the following references which are incorporated by reference herein U.S. Pat. Nos. 2,678,964 to Loughlin; 2,851,522 to Hollywood; 2,863,999 to French; 2,740,071 to Goldmark et al.; and 4,030,121 to Faroudja.
In modern television receivers, a sharpness or peaking control signal is used to boost the level of the auxiliary signal prior to combining it with the video signal. The sharpness signal is for example, a D.C. signal having a relatively constant amplitude and is normally adjusted by the viewer according to individual taste. The boosted auxiliary signal, which is commonly a function signal derived from the video signal, is then added to the video signal to provide an output signal which represents the video signal with shortened transition times. One problem with such a circuit however is that any noise present in the auxiliary signal is also boosted in level.
An ideal, noiseless function signal, for example a second derivative of the luminance signal, would not be boosted during non-transition periods (where it is equal to zero). However, if noise is present during the non-transition periods, it will be amplified as a result of the sharpness signal and will effect the total boosted auxiliary signal at all times and not only during the transition portions of the video signal. Since as a practical matter, some noise will always be present, the viewer When adjusting the amplitude of the sharpness signal, must choose between a sharper picture with increased noise content and a picture having reduced sharpness but less background noise. The prior art concentrates on shortening transition times and minimizing pre-shoot, over-shoot and ringing. It does not however address the problem of amplified noise resulting from a boosted auxiliary signal.
It is an object of the invention therefore, to provide a sharper television picture. It is a further object of the invention to provide a sharper television picture with reduced noise artifacts.